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United States Department of Agriculture

Agricultural Research Service

Title: Re-Evaluation of Paraquat in Assessing Risk of a Candidate Weed Biological Control Agent

Authors
item Cavin, Craig
item Bruckart, William

Submitted to: Proceedings of Northeastern Weed Science Society
Publication Type: Abstract Only
Publication Acceptance Date: December 2, 2004
Publication Date: March 3, 2005
Citation: Proceedings. Northeastern Weed Science Society 60: (in press)

Technical Abstract: Colletotrichum gloeosporioides is the active ingredient in two registered mycoherbicides in North America, 'Collego' for control of northern jointvetch (Aeschynomene virginica) and 'Mallet W.P' for control of roundleaf mallow (Malva pusilla). Each of the isolates is very damaging to its target and does not affect other plants. However, species of Colletotrichum are known also to cause symptomless (or latent) infections that sometimes result in a delayed disease response (i.e., hemibiotrophic). These infections are very difficult to detect and manifestation of the pathogen in infected plants often occurs during senescence or ripening. This phenomenon with Colletotrichum species raises significant issues from the regulatory perspective. The herbicide paraquat (1,1'-dimethyl-4,4'-bipyridinium ion) was reported to induce plant senescence and, for this reason, was considered useful in detecting latent infections of Colletotrichum species in inoculated plants (Cerkauskas, 1988). Cerkauskas proposed that the paraquat procedure be included in risk assessments of Colletotrichum species that are candidates for biological control of weeds. Currently, an isolate of C. gloeosporioides (Cg) from Hungary is under evaluation for biological control of Russian thistle (RT, Salsola tragus) in the U.S. Host range tests suggest the RT Cg isolate is host specific; it does not cause symptoms on a closely related Salsola species or on other non-target plants, does not visibly affect biomass of these plants, and does not sporulate in symptomless tissue, even under moist chamber conditions. However, paraquat treatment of symptomless tissue from three species of unrelated, non-target plants resulted in sporulation by Cg in moist chambers. To understand whether these plants are truly infected without symptoms or if there is another mechanism by which treatment with paraquat leads to manifestation of Cg, soybean plants were inoculated with 10^6 conidia/ml Cg from RT, given dew (18 hr at 25C), incubated in a standard greenhouse, either sampled directly or surface sterilized with a sodium hypochlorite solution (0.5% NaOCl for 5 min), and then treated with paraquat. Leaf disks removed from plants subject to each of the treatments were placed in moist chambers and observed for presence or absence of spore masses from acervuli . Results show that surface sterilization prior to treatment with paraquat greatly reduces the number and rate of formation of acervuli, compared to untreated controls (i.e., treated with paraquat without prior surface sterilization). The current hypothesis is that conidia survive on the leaf surface (i.e., do not infect the plant) and as a saprophyte, colonize tissue killed by the paraquat.

Last Modified: 12/24/2014
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